The present invention relates to improvements in a displacement detecting apparatus which includes an error correcting subsystem.
Generally, a displacement detecting apparatus (scale apparatus) utilizes an interpolation method for dividing a regenerated wave length .lambda. of an incremental signal, in order to have a high-resolution that can distinguish the regenerated wave length .lambda. of an incremental signal. As shown in FIGS. 16A to 16D, when an electromagnetic division (scale) are formed on a scale 1001, signals detected by detection heads 1002 and 1003 are represented as follows: EQU E.sub.A =A sin (2.pi.X/.lambda.), E.sub.B =B cos (2.pi.X/.lambda.)
where A and B are amplitudes of detected signals E.sub.A and E.sub.B, and X is a postion of a detection head.
Assuming that B=A, the equation E.sub.A /E.sub.B =A sin (2.pi.X/.lambda.)/A cos (2.pi.X/.lambda.)=tan(2.pi.X/.lambda.) is derived. Accordingly, the equation X=(.lambda./2.pi.)arctan(E.sub.A /E.sub.B) is obtained. This relationship by this equation forms a Lissajous figure of FIG. 16D. As shown in FIG. 16D, since A=B, the Lissajous figure becomes a circle. Assuming that the position X of the head is (.lambda./2.pi.).theta.(X=(.lambda./2.pi.).theta.), the phase .theta. thereof is represented by (E.sub.A /E.sub.B), that is, .theta.=arctan (E.sub.A /E.sub.B). At this time, the errors of the signals are generated if one of the following conditions is satisfied: a first condition that a direct current component is superposed on the incremental signal as shown in FIGS. 17A to 17C; a second condition that the amplitude of the signal a becomes different from that of the signal b, and a third condition that the phase 30 shift between the signals a and b is not accurately controlled at 90.degree.. Signal values Ea and Eb of such signals a and b can be represented as follows: EQU E.sub.a =A sin (2.pi.X/.lambda.)+D.sub.1, E.sub.b =B sin {(2.pi.X/.lambda.)+.delta.}+D.sub.2
The head position X within one wave length .lambda. is represented as follows: EQU X=(.lambda./2.pi.) arctan (E.sub.a /E.sub.b)
In this case, the Lissajous figure forms an ellipse as shown in FIG. 17C.
In order to eleminate such direct-current components D.sub.1 and D.sub.2, Japanese Patent Provisional Publication No. 2-251720 discloses a correction system as shown in FIG. 18. In this conventional system, signals a and b indicative of a scale detected from a scale 1 are sent to an A/D converter (analog-to-digital converter) wherein the signals a and b are converted into digital signals. The digital signals are sent to a digital signal processing circuit 8 of a CPU (Central Processing Unit) wherein the calculation for a correction is executed. As shown in FIG. 18, the detected signals a and b from detectors 2 and 3 on the scale 1 are supplied to a digital signal processing circuit 8 through amplifiers 4 and 5 and A/D converters 6 and 7. The digital signal processing circuit 8 samples the signals a and b as shown in FIG. 19 and stores them in a RAM 10. The digital signal processing circuit 8 obtains a maximum value and a minimum value of each signal data by comparing the newest data of the signals a and b with the previously stored data. Further, direct-current component (deviations) D.sub.1 and D.sub.2 are obtained by calculating a mid point between the maximum and minimum values of each signal. By subtracting the obtained direct-current components (deviations) D.sub.1 and D.sub.2 from the signals a and b, the digital signal processing circuit 8 generates corrected signals and supplies them to a data conversion circuit 11 wherein A/B phase signal is generated.
The overlapped errors on the detection sensors 2 and 3 are controlled by the above-mentioned circuit. In addition to this error correction, the scale apparatus further required to eliminate various error factors such as a deviation of the direct-current error component due to the external magnetic field as shown in FIG. 20A, an output deviation due to a clearance deviation between a storage medium and the detection head as shown in FIG. 20B and a deviation due to phase deviation caused by the waviness of a head-running standard surface as shown in FIG. 20C. In order to eliminate such errors, the scale apparatus of Japanese Patent Provisional Publication No. 2-251720 is provided with a direct-current component detecting circuit, an output amplitude detecting circuit and a phase detecting circuit for obtaining respective correction values. The scale apparatus including a correction circuit calculates the amount of error from the correction values. The digital signal processing circuit 8 executes a correction of errors according to the direct-current component, an output amplitude difference and a phase shift of the signals a and b. More particularly, the scale apparatus obtains a maximum value and a minimum value of each signal a, b and executes a correction of the signals a and b by calculations. The corrected signals are supplied to an interpolation circuit wherein arc-tangent of the corrected signals are obtained as an interpolation.
However, with this conventional correcting method of the scale apparatus, it is necessary to execute a high-speed sampling for detecting peak values of the signals a and b as shown in FIG. 19 and to frequently execute calculations of a complicated trigonometric function. Therefore, such a displacement detecting apparatus has to provide two high-speed A/D converters and a high-speed performing CPU or digital signal processor. Further, since the signals a and b from the scale is deviated due to the temperature deviation, it is necessary to provide a high-bit and high-accuracy A/D converter. These installations largely increase the cost of the correction circuit and therefore the displacement detecting apparatus including this correction circuit is largely increased in cost.